friction i 11/8/13. bellwork what is the description of friction force? what is the description of...

Friction IFriction I

11/8/1311/8/13

BellworkBellwork

What is the description of friction force?What is the description of friction force? The force exerted by a surface as an object The force exerted by a surface as an object

moves across it or makes an effort to move moves across it or makes an effort to move across it.across it.

AgendaAgenda

Go over H.W.Go over H.W. Sports figure video – “That Mu You Do”Sports figure video – “That Mu You Do” NotesNotes

Friction IFriction I

11/8/1311/8/13

Physics QuestionsPhysics Questions

Answer True or False, in your notes, for the Answer True or False, in your notes, for the following questionsfollowing questions

14 questions total14 questions total You will be answering them by a show of You will be answering them by a show of

handshands

Question #1Question #1

Ben travels to a distant planet. In outer Ben travels to a distant planet. In outer space, with essentially no gravity, he can space, with essentially no gravity, he can throw a baseball (mass = 0.5 kg) and a shot throw a baseball (mass = 0.5 kg) and a shot put (mass = 4 kg) horizontally at the same put (mass = 4 kg) horizontally at the same speed with the same force.speed with the same force.

Question #2Question #2

Gravity in the International Space Station is Gravity in the International Space Station is zero. zero.

Question #3Question #3

The force of gravity on an object is the same The force of gravity on an object is the same as the weight of the object.as the weight of the object.

Question #4Question #4

Mass and weight have essentially the same Mass and weight have essentially the same meaning. To increase the weight of an meaning. To increase the weight of an object requires an increase in the mass of object requires an increase in the mass of the object.the object.

Question #5Question #5

Friction is caused by surface roughness.Friction is caused by surface roughness.

Question #6Question #6

An object will slow down if there is no net An object will slow down if there is no net force. force.

Question #7Question #7

Passive forces exist (tables can exert force). Passive forces exist (tables can exert force).

Question #8Question #8

Surface forces can't exceed the weight Surface forces can't exceed the weight (active force) on an object.(active force) on an object.

Question #9Question #9

An object with a constant net force will An object with a constant net force will have a constant acceleration.have a constant acceleration.

Question #10Question #10

Forces can be negative.Forces can be negative.

Question #11Question #11

Larger (smaller) velocity means larger Larger (smaller) velocity means larger (smaller) acceleration.(smaller) acceleration.

Question #12Question #12

Zero velocity means zero acceleration. Zero velocity means zero acceleration.

Question #13Question #13

Spacecraft need thrusters to keep them Spacecraft need thrusters to keep them going.going.

Question #14Question #14

Objects fall at the same rate because they Objects fall at the same rate because they have the same force of gravity acting on have the same force of gravity acting on them.them.

Question #1Question #1

Ben travels to a distant planet. In outer Ben travels to a distant planet. In outer space, with essentially no gravity, he can space, with essentially no gravity, he can throw a baseball (mass = 0.5 kg) and a shot throw a baseball (mass = 0.5 kg) and a shot put (mass = 4 kg) horizontally at the same put (mass = 4 kg) horizontally at the same speed with no discernable difference in speed with no discernable difference in effort.effort.

FALSE! FALSE! It still will take more force to accelerate a It still will take more force to accelerate a

more massive object.more massive object.

Question #2Question #2

Gravity in the space station is zero. Gravity in the space station is zero. FALSEFALSE The reason that the astronauts act The reason that the astronauts act

weightless is that they're inside a container weightless is that they're inside a container which is FALLING!which is FALLING!

Question #3Question #3

The force of gravity on an object is the same The force of gravity on an object is the same as the weight of the object.as the weight of the object.

TrueTrue Force of gravity is the same as the weight.Force of gravity is the same as the weight.

Question #4Question #4 Mass and weight have essentially the same Mass and weight have essentially the same

meaning. To increase the weight of an meaning. To increase the weight of an object requires an increase in the mass of object requires an increase in the mass of the object.the object.

FALSE, FALSE, WeightWeight is the gravitational is the gravitational attraction force on an object from a nearby attraction force on an object from a nearby planet. planet. MassMass in chemistry is the amount of in chemistry is the amount of matter present. In physics it’s inertia, or in matter present. In physics it’s inertia, or in other words the resistance to changes in other words the resistance to changes in motion.motion.

Question #5Question #5 Friction is caused by surface roughness.Friction is caused by surface roughness. True, Friction is mostly caused by chemical True, Friction is mostly caused by chemical

bonding between the moving surfaces; it is bonding between the moving surfaces; it is caused by stickiness. caused by stickiness.

(When thinking about friction, don't think (When thinking about friction, don't think about grains of sand on sandpaper. Instead about grains of sand on sandpaper. Instead think about sticky adhesive tape being think about sticky adhesive tape being dragged along a surface.)dragged along a surface.)

Question #6Question #6 An object will slow down if there is no net force.An object will slow down if there is no net force. FalseFalse This is the Aristotelian idea that an object's This is the Aristotelian idea that an object's

natural state is rest. Where objects only move natural state is rest. Where objects only move when a net force is exerted upon them. This stems when a net force is exerted upon them. This stems from common everyday observations, e.g. students from common everyday observations, e.g. students seeing that objects which have been pushed across seeing that objects which have been pushed across the floor come to a stop (and not seeing friction as the floor come to a stop (and not seeing friction as a dissipative force acting on the object).a dissipative force acting on the object).

Question #7Question #7

Passive forces exist (tables can exert force). Passive forces exist (tables can exert force). TrueTrue Some students believe that inert objects Some students believe that inert objects

cannot exert a force. They can alter an cannot exert a force. They can alter an object's motion, but they don't exert a force. object's motion, but they don't exert a force. This is untrue, the table DOES exert a This is untrue, the table DOES exert a force.force.

Question #8Question #8

Surface forces can't exceed the weight Surface forces can't exceed the weight (active force) on an object.(active force) on an object.

FalseFalse In acceleration situations surface forces are In acceleration situations surface forces are

greater then the force of gravity, elevator greater then the force of gravity, elevator starting upward, roller coaster at the starting upward, roller coaster at the bottom of a loop, an object hitting the bottom of a loop, an object hitting the ground.ground.

Question #9Question #9

An object with a constant net force will An object with a constant net force will have a constant acceleration.have a constant acceleration.

TrueTrue Constant net force will accelerate an objects Constant net force will accelerate an objects

at a constant rate at a constant rate

Question #10Question #10

Forces can be negative.Forces can be negative. TrueTrue The negative means direction (down, left)The negative means direction (down, left)

Question #11Question #11

Larger (smaller) velocity means larger Larger (smaller) velocity means larger (smaller) acceleration.(smaller) acceleration.

FalseFalse A small velocity that stops quickly has a A small velocity that stops quickly has a

high accelerationhigh acceleration A large velocity that takes a long time to A large velocity that takes a long time to

stop has a low accelerationstop has a low acceleration

Question #12Question #12

Zero velocity means zero acceleration. Zero velocity means zero acceleration. FalseFalse A ball thrown straight up at the top of it A ball thrown straight up at the top of it

path has zero velocity, but is accelerating do path has zero velocity, but is accelerating do to gravity (-9.81 m/sto gravity (-9.81 m/s22))

Question #13Question #13

Spacecraft need thrusters to keep them Spacecraft need thrusters to keep them going.going.

FalseFalse Newton’s 1Newton’s 1stst Law – An object in motion will Law – An object in motion will

stay in motion stay in motion

Question #14Question #14

Objects fall at the same rate because they Objects fall at the same rate because they have the same force of gravity acting on have the same force of gravity acting on them.them.

False!False! So what is the reason? (Will be answered So what is the reason? (Will be answered

shortly.)shortly.)

Mass vs. WeightMass vs. Weight

Mass (scalar)Mass (scalar): Is the amount of matter an : Is the amount of matter an object has object has Unit is the kilogram (kg)Unit is the kilogram (kg)

Weight (vector)Weight (vector): The force of gravity that : The force of gravity that acts on an object acts on an object

Unit is the Newton (kg · m/sUnit is the Newton (kg · m/s22)) Mass and weight are two different Mass and weight are two different

quantities! quantities!

Coefficient of FrictionCoefficient of Friction

Coefficient of Friction (scalar)Coefficient of Friction (scalar): The ratio : The ratio between the force necessary to move one between the force necessary to move one surface horizontally over another. surface horizontally over another. It is It is reduced once the motion has started.reduced once the motion has started.

Unit is a ratio (unitless)Unit is a ratio (unitless)

Types of FrictionTypes of Friction

Static FrictionStatic Friction – – is the friction force is the friction force resisting the relative motion of two surfaces resisting the relative motion of two surfaces that are not movingthat are not moving

Kinetic FrictionKinetic Friction – is the friction force of two – is the friction force of two surfaces that are sliding past each othersurfaces that are sliding past each other

Inertia Inertia

InertiaInertia (scalar): The resistance of a body or (scalar): The resistance of a body or object to change its motion.object to change its motion.

Mass is the measurement of inertia (kg) Mass is the measurement of inertia (kg)

Force Quantities

Quantity Type Unit

Acceleration Vector m/s2

Acceleration of gravity

Vector m/s2

Force of gravity Vector N (kg m / s2)

Mass Scalar kg

Coefficient of Friction

Scalar (ratio)

Mythbusters – Phonebook FrictionMythbusters – Phonebook Friction

http://www.youtube.com/watch?v=AX_lCOjLhttp://www.youtube.com/watch?v=AX_lCOjLCToCTo

http://www.youtube.com/watch?v=QMW_uYhttp://www.youtube.com/watch?v=QMW_uYWwHWQ&feature=relatedWwHWQ&feature=related

http://http://www.youtube.com/watch?vwww.youtube.com/watch?v=hG7lGZqWFpM=hG7lGZqWFpM